Two issues that currently exist in wireless broadband systems are latency of random access (RA) failure detection on the subscriber side and collisions. An RA attempt fails when the BS fails to receive the RA signal correctly either due to collision, i.e., two or more users (subscribers) use the same RA opportunity, or due to an insufficient signal level.
The first issue is the latency issue of RA failure detection on the subscriber side. In scheduling-based access systems, RA accommodates new access needs, e.g., new users, existing users' new requirements, etc. Broadband Wireless Access systems, e.g., worldwide interoperablility for microwave access (WiMAX), and long term evolution (LTE), are typical scheduling-based access systems, where a base station (BS) controls the use of the air link resources.
When a RA failure occurs, the subscriber needs to detect it and then take actions accordingly, e.g., retry, or ramp up its transmission power. A commonly-used mechanism for a subscriber to detect the failure of its random access attempt is timer-based, i.e., after waiting a pre-defined time period without getting the expected response, the subscriber assumes that its previous RA attempt failed, where the expected response depends on the purpose of the RA.
The pre-defined time period may play a role in a timer-based RA failure detection mechanism because the recovery from a RA failure has to wait for the pre-defined time that needs to be sufficiently long enough to process the RA request receptions and responses under the worst-case considerations, e.g., the heaviest traffic loading.
The second issue relates to collision scenarios. A collision occurs when two or more subscribers choose the same RA opportunity, where a RA opportunity refers to the opportunity for a subscriber to send a RA request. For example, a RA opportunity in IEEE 802.16 systems consists of a RA channel and a RA code sent on the RA channel. When a collision occurs, there are several possible results. The first result is that the BS detects nothing. The second result is that the BS detects a collision. The third result is that a single RA Request is erroneously detected by the BS in the RA opportunity with collision.